Chapter 3: Telescopes

Images can be formed through reflection or refraction.

Reflecting mirror

3.1 Optical Telescopes

Image formation

Modern telescopes are all reflectors:

• Light traveling through lens is refracted differently depending on wavelength.

• Some light traveling through lens is absorbed.

• Large lens can be very heavy, and can only be supported at edge.

• Lens needs two optically acceptable surfaces, mirror only needs one.

Types of reflecting telescopes

Details of the Keck telescope

Image acquisition: Charge-coupled devices (CCDs) are electronic devices, can be quickly read out and reset.

Image processing by computers can sharpen images.

Light-gathering power:

Improves detail

Brightness proportional to square of radius of mirror

The figure, part (b) was taken with a telescope twice the size of (a)

3.2 Telescope Size

The VLT (Very Large Telescope), Atacama, Chile

Effect of improving resolution:

(a) 10 ; (b) 1 ; (c) 5″′ ′ ; (d) 1″

Atmospheric blurring due to air movements

3.3 High-Resolution Astronomy

Solutions:• Put telescopes on mountaintops, especially in deserts.• Put telescopes in space.• Active optics – control mirrors based on temperature and orientation.

Radio telescopes:• Similar to optical reflecting telescopes• Prime focus• Less sensitive to imperfections (due to longer wavelength); can be made very large

3.4 Radio Astronomy

Largest radio telescope: 300-m dish at Arecibo

Longer wavelength means poorer angular resolution.

Advantages of radio astronomy:

• Can observe 24 hours a day.

• Clouds, rain, and snow don’t interfere.

• Observations at an entirely different frequency; get totally different information.

Interferometry:• Combines information from several widely spread radio telescopes as if it came from a single dish.

• Resolution will be that of dish whose diameter = largest separation between dishes.

Interferometry requires preserving the phase relationship between waves over the distance between individual telescopes.

Can get radio images whose resolution is close to optical.

Infrared radiation can image where visible radiation is blocked; generally can use optical telescope mirrors and lenses.

3.5 Other Astronomies

Infrared telescopes can also be in space or flown on balloons.

Ultraviolet images

(a)The Cygnus loop supernova remnant

(b) M81

X rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniques.X rays will reflect at a very shallow angle, and can therefore be focused.

X-ray image of supernova remnant Cassiopeia A

Gamma rays cannot be focused at all; images are therefore coarse.

Much can be learned from observing the same astronomical object at many wavelengths. Here, the Milky Way.

What is the most important function of a telescope?

A. MagnificationB. ResolutionC. Light collectionD. None of the above 

The most serious disadvantage of refracting telescopes is

A. Their extreme expense.B. Their awkward size.C. The inconvenient location of the

focused image.D. Distortion of the image due to

sagging and bending.

Astronomers wish to build larger and larger telescopes because larger telescopes

A. allow fainter objects to be seen.B. allow more detail in the image to

be seen or photographed.C. produce greater magnification of

the image.D. allow more wavelengths to be

studied.E. both (a) and (b). 

Which spectral region produces the poorest resolution?

A. X-rays.B. Radio waves.C. Gamma raysD. Visible light.E. None of the above.

In which two spectral regions are ground based observations MOST useful?

A. Microwave and ultraviolet.B. Optical and X-ray.C. Radio and optical.D. Ultraviolet and infrared.

• Refracting telescopes make images with a lens.

• Reflecting telescopes make images with a mirror.

• Modern research telescopes are all reflectors.

• CCDs are used for data collection.

• Data can be formed into image, analyzed spectroscopically, or used to measure intensity.

• Large telescopes gather much more light, allowing study of very faint sources.

• Large telescopes also have better resolution.

Summary of Chapter 3

• Resolution of ground-based optical telescopes is limited by atmospheric effects.

• Resolution of radio or space-based telescopes is limited by diffraction.

• Active and adaptive optics can minimize atmospheric effects.

• Radio telescopes need large collection area; diffraction is limited.

• Interferometry can greatly improve resolution.

Summary of Chapter 3, cont.

• Infrared and ultraviolet telescopes are similar to optical.

• Ultraviolet telescopes must be above atmosphere.

• X rays can be focused, but very differently than visible light.

• Gamma rays can be detected but not imaged.

Summary of Chapter 3, cont.